Permeable membranes are known to separate or selectively enrich a gas mixture. For example, membranes are used in the separation of H.sub.2 from supercritical gases such as N.sub.2, CO and CH.sub.4 ; the separation of CO.sub.2 and water vapor; and the enrichment of air by nitrogen or oxygen. Hydrogen is recovered from ammonia production plants using large scale membrane technology.
The permeability, P, of a gaseous penetrant can be written as the product of an effective solubility of the penetrant in the polymer matrix, S, and an average diffusivity of the penetrant through the polymer matrix, D. EQU P=D S (1)
The average penetrant diffusivity, D, can be determined by dividing the penetrant's permeability by the penetrant's solubility coefficient. With negligible downstream pressure, the solubility coefficient, S, is equal to the secant slope of the gas sorption isotherm evaluated at the upstream conditions. The solubility coefficient is thermodynamic in nature, and is affected by the inherent condensability of the penetrant, polymer-penetrant interactions, and the amount of excess or free volume existing in the glassy polymer. Increases in either D or S will tend to increase the permeability coefficient; however, permselectivity must also be considered in structural modifications to avoid undesirable trade-offs between productivity and selectivity.
Since the downstream pressure is nearly zero, the gas separation factor for a mixture of gases A and B, .alpha..sub.A/B, defined by eqn. (2) is rigorously equal to the ideal separation factor based on the individual permeabilities of the two gases A and B, .alpha.*.sub.A/B, given by eqn. (3): ##EQU1## where x.sub.i 's and y.sub.i 's are the mole fractions of components A and B at the upstream and downstream sides of the membrane, respectively. The ideal separation factor, .alpha.*.sub.A/B, provides a measure of the intrinsic permselectivity of a membrane material for mixtures of A and B. In the absence of plasticizing effects due to strong polymer-penetrant interactions, .alpha.*.sub.A/B in mixed gas situations can be approximated to within about 10% using the more easily measured ratio of permeabilities of pure gases A and B.
If eqn. (1) is substituted into eqn. (3), the ideal separation factor can be separated into two parts: ##EQU2## where D.sub.A /D.sub.B is the diffusivity selectivity, and S.sub.A /S.sub.B is the solubility selectivity. The diffusivity selectivity is determined by the ability of the polymer to discriminate between the penetrants on the basis of their sizes and shapes, and is governed by intrasegmental motions and intersegmental packing. The importance of steric factors is demonstrated by the greater diffusivity of carbon dioxide over methane despite the fact that methane has the lower molecular weight of the two molecules. The solubility selectivity, like the solubility, is thermodynamic in nature. Experience has shown that this factor is difficult to adjust without causing significant losses in the diffusivity selectivity. The key to structure-property optimization for gas separation membranes, then, is finding means to increase the diffusivity selectivity without incurring large reductions in the diffusivity of the desired component, A.
Significant increases in diffusivity and diffusivity selectivity can be obtained by simultaneously inhibiting intrasegmental motions and intersegmental chain packing. These results can be simmarized as two principles for tailoring membrane materials:
1. structural moieties which inhibit chain packing while simultaneously inhibiting torsional motion about flexible linkages on the polymer backbone tend to increase permeability while maintaining permselectivity; PA1 2. structural moieties which decrease the concentration of mobile linkages in the polymer backbone and do not significantly change intersegmental packing tend to increase permselectivity without decreasing permeability significantly.
The ratio of specific free volume to polymer specific volume, the fractional free volume, is representative of the degree of openness of the matrix. This index takes into account the filling of space by bulky side groups, but is not experimentally determined. An estimate of the occupied volume of the polymer is made by using a group contribution method such as that of Bondi (1968) or Van Krevelen and Hoftyzer (1976).
Much of the work in the field has been directed to developing membranes which optimize the separation factor and total flux of a given system. It is disclosed in U.S. Pat. No. 4,717,394 to Hayes that aromatic polyimides containing the residue of alkylated aromatic diamines are useful in separating a variety of gases. Moreover, it has been reported in the literature that other polyimides, polycarbonates, polyurethanes, polysulfones and polyphenyleneoxides are useful for like purposes.
U.S. Pat. No. 5,074,891 to Kohn et al. discloses certain polyimides with the residuum of a diaryl fluorine-containing diamine moiety as useful in separation processes involving, for example, H.sub.2, N.sub.2, CH.sub.4, CO, CO.sub.2, He and O.sub.2.
By utilizing a more rigid repeat unit than a polyimide, however, even greater permeability and permselectivity are realized. Such a rigid repeat unit is a polypyrrolone.
Polypyrrolones are condensation polymers obtained from the reaction of aromatic dianhydrides and aromatic tetraamines followed by complete cyclization. The polymer obtained by the initial reaction of the monomers in an aprotic solvent is a soluble poly(amide amino acid), which can be thermally cyclized. The polypyrrolone resulting from cyclization possesses a repeat unit with two benzene rings joined by two fused five membered rings, imparting a great degree of thermal and chemical resistance, strength and rigidity. The rigidity of the polypyrrolone repeat unit provides unusually high size and shape discrimination between the penetrants. With the incorporation of the proper linkages in the repeat unit, the intrinsic rigidity of the polypyrrolone linkage can also inhibit packing, allowing one to increase penetrant mobility without losses in selectivity.
Polypyrrolones as membrane materials were proposed and studied originally for the reverse osmosis purification of water by H. Scott et al. (1970).
The syntheses, permeabilities, solubilities and diffusivities of the polyimides, 6FDA-6F.sub.p DA, 6FDA-IPDA, 6FDA-ODA, 6FDA-DAF and the polypyrrolone 6FDA-TADPO have been described (Walker and Koros, 1991; Koros and Walker, 1991; Kim et al. 1988a, b; Kim 1988c; Coleman, 1992). Their structures, permeabilities, solubilities, diffusivities, glass transition temperatures and fractional free volumes are shown in Tables 1-10.
TABLE 1 __________________________________________________________________________ Pure Gas Permeabilities and Ideal Permselectivities for Polyimides. Permeabilities are in units of 10.sup.-10 (cc(STP) cm/cm.sup.2 sec cmHg) = 1 Barrer He O.sub.2 CO.sub.2 Polymer P.sub.He P.sub.O.sbsb.2 P.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 __________________________________________________________________________ 6FDA-ODA 51.5 4.34 23.0 135.4 5.2 60.5 ##STR1## 6-FDA-IPDA 71.2 7.53 30.0 102.1 5.6 42.9 ##STR2## 6FDA-DAF 98.5 7.85 32.2 156.3 6.2 51.1 ##STR3## 6FDA-6FpDA 137 16.3 63.9 85.6 4.7 39.9 ##STR4## __________________________________________________________________________
TABLE 2 __________________________________________________________________________ Pure Gas Permeabilities and Ideal Permselectivities for the 6FDA-TADPO Polyamide. Permeabilities are in units of 10.sup.-10 (cc(STP) cm/cm.sup.2 sec cmHg) = 1 Barrer He O.sub.2 CO.sub.2 Polymer P.sub.He P.sub.O.sbsb.2 P.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 __________________________________________________________________________ 6FDA-TADPO 19.1 0.97 3.69 230.9 5.8 44.5 ##STR5## __________________________________________________________________________
TABLE 3 __________________________________________________________________________ Pure Gas Permeabilities and Ideal Permselectivities for the 6FDA-TADPO Polypyrrolone. Permeabilities are in units of 10.sup.-10 (cc(STP) cm/cm.sup.2 sec cmHg) = 1 Barrer He O.sub.2 CO.sub.2 Polymer P.sub.He P.sub.O.sbsb.2 P.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 __________________________________________________________________________ 6FDA-TADPO 90.2 7.9 27.4 171.5 6.5 52.2 ##STR6## __________________________________________________________________________
TABLE 4 __________________________________________________________________________ Pure Gas Solubilities and Solubility Selectivities for Polyimides. Solubilities are in units of (cc(STP)/cc atm). He O.sub.2 CO.sub.2 Polymer S.sub.He S.sub.O.sbsb.2 S.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 __________________________________________________________________________ 6FDA-ODA 0.079 1.03 4.89 0.060 1.90 3.70 ##STR7## 6FDA-IPDA 0.079 0.9 4.24 0.066 1.81 3.53 ##STR8## 6FDA-DAF 0.096 1.20 5.02 0.060 1.78 3.14 ##STR9## 6FDA-6FpDA 0.072 0.99 5.99 0.051 1.48 4.16 ##STR10## __________________________________________________________________________
TABLE 5 __________________________________________________________________________ Pure Gas Solubilities and Solubility Selectivities for the 6FDA-TADPO Polyamide. Solubilities are in units of (cc(STP)/cc atm). He O.sub.2 CO.sub.2 Polymer S.sub.He S.sub.O.sbsb.2 S.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 __________________________________________________________________________ 6FDA-TADPO 0.067 0.55 3.06 0.085 1.46 3.87 ##STR11## __________________________________________________________________________
TABLE 6 __________________________________________________________________________ Pure Gas Solubilities and Solubility Selectivities for the 6FDA-TADPO Polypyrrolone. Solubilities are in units of (cc(STP)/cc atm). He O.sub.2 CO.sub.2 Polymer S.sub.He S.sub.O.sbsb.2 S.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 __________________________________________________________________________ 6FDA-TADPO 0.103 1.09 4.65 0.060 1.161 2.71 ##STR12## __________________________________________________________________________
TABLE 7 __________________________________________________________________________ Pure Gas Diffusivities and Diffusivity Selectivities for Polyimides. Diffusivities are in units of 10.sup.-10 cm.sup.2 /sec. He O.sub.2 CO.sub.2 Polymer D.sub.He D.sub.O.sbsb.2 D.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 __________________________________________________________________________ 6FDA-ODA 49500 320 358 2262 2.75 16.3 ##STR13## 6FDA-IPDA 68500 636 538 1546 3.14 12.1 ##STR14## 6FDA-DAF 78000 497 488 2608 3.47 16.3 ##STR15## 6FDA-6FpDA 145000 1250 811 1678 3.18 9.6 ##STR16## __________________________________________________________________________
TABLE 8 ______________________________________ Pure Gas Diffusivities and Diffusivity Selectivities for the 6FDA-TADPO Polyamide. Diffusivities are in units of 10.sup.-10 cm.sup.2 /sec. He O.sub.2 CO.sub.2 Polymer D.sub.He D.sub.O.sbsb.2 D.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 ______________________________________ 6FDA-TADPO 21600 135 92 2708 4.01 11.5 ##STR17## ______________________________________
TABLE 9 __________________________________________________________________________ Pure Gas Diffusivities and Diffusivity Selectivities for the 6FDA-TADPO Polypyrrolone. Diffusitivities are in units of 10.sup.-10 cm.sup.2 /sec. He O.sub.2 CO.sub.2 Polymer D.sub.He D.sub.O.sbsb.2 D.sub.CO.sbsb.2 CH.sub.4 N.sub.2 CH.sub.4 __________________________________________________________________________ 6-FDA-TADPO 66500 549 448 2858 5.9 19.3 ##STR18## __________________________________________________________________________
TABLE 10 ______________________________________ Glass Transition Temperatures and Fractional Free Volumes for Polyimides, Polyamides and Polypyrrolones Glass Transition Fractional Free Polymer Tg, .degree.C. Volme ______________________________________ Properties of the Polyimides. 6FDA-ODA 304 0.1635 6FDA-IPDA 310 0.1680 6FDA-DAF 394 0.1588 6FDA-6FpDA 320 0.1897 Properties of the Polyamides 6FDA-TADPO Ring Closure 0.1476 Properties of the Polypyrrolones. 6FDA-TADPO .about.375 0.1956 ______________________________________
The 6FDA-TADPO polypyrrolone shows attractive transport properties, high permeabilities and good selectivities for all gas pairs studied. The 6FDA-TADPO polypyrrolone is structurally similar to -the 6FDA-ODA polyamide. Both polymers have an ether linkage and a hexafluoroisopropylidene linkage as part of their repeat units. As shown in Tables 1 and 3 of pure gas measurements, the 6FDA-TADPO polypyrrolone is more permeable than the 6FDA-ODA polyimide and is more selective with the exception of the carbon dioxide/methane gas pair.
The 6FDA-TADPO polypyrrolone has the highest diffusivity selectivity for all pure gas pairs, the highest glass transition temperature and the greatest free fractional volume. In the case of mixed gas permeation of carbon dioxide and methane, the 6FDA-TADPO is more selective (FIG. 10).
Once the densities of polymers are known, the specific free volumes and specific volumes can be used to estimate the fractional free volume. The fractional free volume is the ratio of the specific free volume to the specific volume of the polymer. The specific free volume is estimated by a group contribution method and the specific volume is determined by dividing the molecular weight of the repeat unit by the bulk polymer density.
The fractional free volume gives a measure of the degree of openness of the polymeric matrix. Materials with larger fractional free volumes have greater diffusivities and thus greater permeabilities than materials with smaller fractional free volumes. The polypyrrolones have fractional free volumes that are greater than the polyimides, so the diffusivities are greater than the polyimides.
The fractional free volume does not give any indication as to the selectivity of a polymer. Within a given polymer family however, polymers with greater fractional free volumes may have lower diffusivity selectivities and thus lower permselectivities. The repeat unit of a polypyrrolone is more rigid than a structurally- similar polyimide, as given by the geometry of the fused ring system and the higher glass transition temperature shown in table 10. Given the greater rigidity of the polypyrrolones, polypyrrolones with fractional free volumes similar to polyimides may have greater diffusivity selectivities and thus greater permselectivities.
The 6FDA-TADPO has half the carbonyl concentration per repeat unit as the 6FDA-ODA, effecting a lower solubility of carbon dioxide in the polypyrrolone. The lower solubility of carbon dioxide together with the rigid fused ring system of the polypyrrolone repeat unit resulted in a greater resistance to penetrant induced permeability increases and selectivity decreases referred to as plasticization.
Plasticization is caused by highly sorbing penetrants, such as carbon dioxide at higher pressures and leads to a significant deleterious loss in permselectivity. The presence of the highly sorbing penetrants leads to a softening of the polymer matrix, much as a liquid solvent or swelling agent would. The loss in permselectivity may also be accompanied by a simultaneous increase in permeability of one or more of the feed components.
The 6FDA-TADPO polypyrrolone and the 6FDA-DAF polyimide are structurally similar with both repeat units having a rigid, flat fused ring system. The transport properties of the polyimide and the polypyrrolone are similar.
The greater permeability of the polypyrrolone can be attributed to the greater fractional free volume of the polypyrrolone. The inherent rigidity of the fused ring system and the absence of a rotatable bond which is present in the polyimides accounts for the greater selectivity of the polypyrrolone.
The present invention provides for polyamide and polypyrrolone membranes for fluid separation with superior permeabilities, solubilities and diffusivities as compared to known materials.
______________________________________ LIST OF ABBREVIATIONS ______________________________________ 6FDA Hexafluoroisopropylidene-bisphthalic anhydride. 6FTA Hexafluorotetraamine ODA 4,4'-Oxydianiline DAF 2,7-Diaminofluorene 6FpDA 4,4'-(Hexafluoroisopropylidene)-Dianiline IPDA 4,4'-(Isopropylidene)-Dianiline TADPO 3,3',4,4'-Tetraaminodiphenyl ether. TABP 3,3',4,4'-Tetraaminobiphenyl. TADPIP 3,3',4,4'-Tetraaminodiphenyl isopropylidene. Polyamide Initial form of polymer from condensation reaction of dianhydride and tetraamine. Polypyrrolone Final form of polymer after thermal curing to 285-300.degree. C. ______________________________________